The present invention is related to high index, low dispersion, low density and fast darkening and fading photochromic glasses for ophthalmic lenses.
In the present context, fast darkening and fading glasses means glasses which show at room temperatures after 15 minutes' exposure to an actinic illumination, a luminous transmission lower or equal to 40% and a luminous transmission greater than about 55% five minutes after they have been removed from the actinic source.
Most of prior art patents dealing with photochromic glasses describe compositions to obtain products, for ophthalmic applications or not, having a refractive index equal to about 1.523.
Ophthalmic lenses of high refractive index present numerous advantages. In fact, using a high index glass instead of a standard glass (n.sub.D =1.523) allows, for a given power, a thickness reduction of the edge (negative power) or of the center (positive power).
Increasing the index of refraction leads generally to a decrease of the Abbe number (or an increase of glass dispersion). In order to minimize the defects induced by the increase of the glass dispersion, mainly colored fringes at the edge of the lens, the glass must have a high Abbe number.
Associated with thickness reduction, another significant advantage can be a lower weight. For that the glass must display a low density; a density less than or equal to 2.80 g/cm.sup.3 is necessary.
The objective of this invention is to combine these advantages of a high refractive index glass, including the above-mentioned criteria, with the advantages of a photochromic glass. This has become a necessity because within a few years the use of glasses with a 1.6 index of refraction will tend to replace for ophthalmic applications the current glasses having a refractive index of 1.523. Thus, the wearer of corrective eye-glasses would get simultaneously the comfort given by a high index lens and the functionality of a photochromic glass.